Since a traction drive type continuously variable transmission (CVT) used for an automobile has a large torque transmission capacity and is used under severe conditions, it is indispensable from the viewpoint of power transmission that a traction oil to be used in a CVT has a traction coefficient which is sufficiently higher than a lowest value in the range of working temperature, that is, the traction coefficient at a high temperature (140° C.) is sufficiently higher than a design value of a CVT.
The traction oil, which also plays a role as an ordinary lubricating oil in a CVT, is required to have high viscosity capable of maintaining sufficient oil films even at a high temperature to prevent friction and/or wear and at the same time, to have low viscosity even at a low temperature for the sake of low temperature startup in cold districts such as North America and North Europe. That is to say, it is necessary that the traction oil has a high viscosity index minimized in variation in viscosity with temperature.
On the basis of the understanding of the above-mentioned background, the present inventors developed a high performance base oil which is for a traction oil and which has a high traction coefficient at a high temperature and a high viscosity index, said high performance has never heretofore been realized {Japanese Patent Application Laid-Open No. 17280/2000 (Heisei 12)}, but from the aspect of practical application, there was need to further add a viscosity index improver to the base oil thus developed. However, since even a small amount of addition of a previously known viscosity index improvers markedly lowers traction coefficient at a high temperature, it has been eagerly desired to develop a viscosity index improver which can improve viscosity index without lowering the traction coefficient of a base oil and which is well suited for traction oils.
There is disclosed in Japanese Patent Application Laid-Open No. 19698/1986 (Showa 61), a traction oil composition which comprises a base oil and a hydrogenation product of a thermoplastic resin bearing an aromatic ring blended therewith, and which is enhanced in viscosity index and traction coefficient. However, it is impossible to judge whether the composition is excellent or not from the practical application, since it is highly volatile owing to a low molecular weight and is composed of practically unsuitable bicyclohexyl compound as a base oil.
There is disclosed polymethacrylate having an alicyclic structure as a viscosity index improver for traction oils {for instance, refer to Japanese Patent Application Laid-Open No. 19697/1986 (Showa 61), Japanese Examined Patent Application Publication No. 62984/1994 (Heisei 6) and Japanese Patent Application Laid-Open No. 19371/2002 (Heisei 14)}. Nevertheless it is difficult to use the same as the case may be, in traction oils under severe working conditions because of insufficient shear stability.